This invention relates generally to phase modulation for fiber optic rate sensors. More particularly, this invention relates to a fiber optic rate sensor having a digital phase modulation capability, thereby eliminating the need for digital to analog conversion equipment as has heretofore been necessary.
The Pockels electro-optical effect operating in the transverse mode can be used to advantage in fiber optic angular rate sensors (gyroscopes) to provide light path phase modulating devices. These devices, i.e. integrated optics chips (IOC's) can be designed to exhibit a linear relationship between the level of an applied control voltage and the phase delay imparted to a confined light path. Typically, these devices use metallized pads on a prepared substrate. The pads are sized and positioned to provide a field strength level and orientation consistent with the intended modulation requirements. The present invention contemplates the use of specific metallized patterns of pads deposited on an optical crystal substrate to provide the digital phase modulation capability.
Lithium niobate and lithium tantalate are generally considered to be the most versatile crystals for all electro-optic applications. High optical quality lithium niobate and lithium tantalate both use the Pockels effect in the transverse voltage mode as light switches for phase modulating devices. In regard to the characteristics and applications of lithium niobate and lithium tantalate, reference is made to the data sheet issued in November, 1985 by Crystal Technology, Inc. of Palo Alto, Calif.
The Pockels effect is described at page 265 of the text "Optics" by Eugene Hecht and Alfred Zajoc, published by Addison-Wesley Publishing Company.
In a particular configuration, the effective index of refraction of a confined optical path is increased by the application of a voltage across two pads that straddle the confined optical path. The field pattern within a crystal substrate of lithium niobate, for example, resulting from this gradient is impressed across the axis of the confined propagation path. An arrangement such as this can be optimized for a nominal center frequency and bandwidth which produces a voltage related effect on the phase of light waves so as to be highly useable in fiber optics applications. One such use would be in fiber optic angular rate sensor or gyroscope (gyro) applications, where it is possible to null out the Sagnac induced phase changes produced in a fiber optic coil under the influence of a rotational rate.
Integrated optics chips of the type described have the ability to shift light phase in response to digital commands. The complexity of a separate digital to analog converter arrangement is undesirable, since it results in larger packages and noise sensitivity while decreasing reliability and performance stability.
Thus, the primary object of the present invention is to provide an arrangement for easily achieving a desireable digital phase modulation capability within an integrated optics chip without resorting to separate digital to analog voltage converter apparatus.